1. Field of the Invention
This invention relates to the detection of oxides of nitrogen and instruments used to effect such determinations.
2. Description of the Prior Art
In various environments, nitrogen oxides (NO.sub.x), i.e. NO and NO.sub.2, may be present as the result of blasting or the operation of diesel engines. These gases can pose special problems in mines where the regulation of the atmosphere is not easily controlled. The current threshold limit value (TLV) for NO.sub.2 is 5 parts per million (ppm) while that for NO is 25 ppm. The TLV is defined as the time-weighted average for exposure over a 40 hour work week. Thus, as established by the American Conference of Governmental Industrial Hygiensts in 1973, exposures to materials at levels above the TLV value are permissible for short periods if the time-weighted average is below the TLV value.
However, a ceiling (C) value has been established for NO.sub.2 at 5 ppm whereby even short exposures to levels above the C value are not acceptable.
A small, pocket-type indicator for use in the field has been disclosed as described in U.S. Pat. No. 3,681,027. The indicator of this reference utilizes color changes in order to alert the wearer of unacceptable concentrations of NO.sub.2 in the atmosphere. However, these indicators essentially only provide for instant determinations and are not acceptable in order to quantitatively assess the wearer's exposure over extended periods such that an accurate time-weighted average of the level of exposure can be established. Thus, these indicators would not find utilization in the determination of whether the wearer had been exposed to levels which were over the TLV value for NO.sub.2. Additionally, although exposures to NO.sub.2 are generally more of a problem than exposure to NO in view of the lower TLV of NO.sub.2 as well as the fact that NO.sub.2 has a ceiling value, determination of the level of exposure to NO can be desirable but is not provided by the indicators of U.S. Pat. No. 3,681,027.
A further example of a dosimeter for the detection of NO.sub.2 is disclosed in U.S. Pat. No. 3,574,552. However, the dosimeter strips which are prepared according to this reference also depend on the occurrence of color changes to determine the exposure levels. Thus, the ability of a wearer to determine the exposure level would depend on the wearer's ability to correlate the color produced by the dosimeter strip with colors produced by exposure to known levels. Additionally, such dosimeter strips would not be particularly useful in the accurate determination of the time-weighted integrated exposure of the wearer. This would result in an inability to accurately determine whether the wearer had been exposed to levels of the NO.sub.2 gas above the TLV value. Further, U.S. Pat. No. 3,574,552 does not provide for the determination of exposure to NO gas.
The use of detector tubes for the determination of levels of various gases is also known as described in U.S. Pat. No. 3,068,073 which describes a detector tube for the determination of carbon dioxide gas. In the use of detector tubes, a material is provided which changes color on exposure to a particular gas, in a transparent tube which is sealed at both ends prior to the determination of the level of the gas present. When the determination of the level present is required, the seals are broken and a known volume of gas is pumped through the tube and any color change as well as the length of such color in the tube is noted. However, the testing must be done at the site in question and, therefore, the presence of a pump may prove cumbersome and undesirable. Additionally, such methods cannot provide integrated time-weighted exposures and are dependent upon the ability of the operator to accurately distinguish changes in color. Further, the accuracy of such stainlength detector tubes is limited and can give a reading 25 to 50% above or below the actual level present.
Instrumental methods which utilize chemiluminescent or electrochemical methods are known but require the utilization of expensive, fragile and bulky instruments. Further, such methods require pumps with accompanying batteries and/or line current. Such instruments can utilize sophisticated electronic circuits which may require servicing and maintenance. Further, instruments of this type may require user manipulation for calibration and adjustment which is inconvenient in many applications for on-site determinations.
Finally, the phenoldisulfonic acid method is known for the determination of total oxides of nitrogen. However, such a determination has a disadvantage in that NO cannot be distinguished from NO.sub.2. Since NO.sub.2 has a significantly lower TLV than NO as well as a ceiling value, this method may not prove to be adequate in various applications. This method is described in "Determination of the Oxides of Nitrogen by the Phenoldisulfonic Acid Method" by Beatty et al., Bureau of Mines Report of Investigations 3687 (February, 1943) and in "Rapid Determination of Nitrogen Oxides with Use of Phenoldisulfonic Acid", by Coulehan et al., Environmental Science and Technology, Volume 5, No. 2, page 163 (February, 1971). This method requires a fragile glass bottle for sample collection and the analysis is lengthy and involved, requiring a chemist or highly-trained technician to operate elaborate equipment.